The present invention is directed to a method and nozzle for producing a coating using a kinetic spray system with much lower powder pressures than previously used. The invention permits one to significantly decrease the cost of the powder delivery system, to run the system at higher temperatures for increased deposition efficiency and to eliminate clogging of the nozzle.
A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled xe2x80x9cKinetic Spray Coatings,xe2x80x9d published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity air stream by the drag effect. The air used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. It is believed that the particles must exceed a critical velocity prior to their being able to bond to the substrate. The critical velocity is dependent on the material of the particle. It is believed that the initial particles to adhere to a metal or alloy substrate have broken the oxide shell on the substrate material permitting subsequent metal to metal bond formation between plastically deformed particles and the substrate. Once an initial layer of particles has been formed on a substrate the subsequent particles both bind to the voids between previously bound particles and also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the air stream because the temperature of the air stream and the time of exposure to the heated air are selected to ensure that the temperature of the particles is always below their melting temperature.
That work had improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.
The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov""s apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns and up to about 106 microns.
The modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings are disclosed in U.S. Pat. Nos. 6,139,913, and 6,283,386. The process and apparatus provide for heating a high pressure air flow up to about 650xc2x0 C. and combining this with a flow of particles. The heated air and particles are directed through a de Laval-type nozzle to produce a particle exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, no phase transition occurs in the particles prior to impingement. It has been found that each type of particle material has a threshold critical velocity that must be exceeded before the material begins to adhere to the substrate. The disclosed method did not disclose the use of particles in excess of 106 microns.
There are several difficulties associated with current kinetic spray systems. First, the powder is injected into the heated main gas stream prior to passage through the de Laval nozzle. Because the heated main gas stream is under high pressure injection of the powder requires high pressure powder delivery systems, which are quite expensive. Second, the powder particles and heated main gas both must pass through the throat of the nozzle and the particles frequently plug a portion of the diverging section and the nozzle throat, which requires a complete shutdown of the system and cleaning of the nozzle. Finally, for a given material the main gas temperature must be sufficiently low that it does not result in melting of the particles and significant plugging of the nozzle, which may not be an ideal temperature for efficient deposition.
In one embodiment the present invention is a method of kinetic spray coating a substrate comprising the steps of: providing particles of a material to be sprayed; providing a supersonic nozzle having a throat located between a converging region and a diverging region; directing a flow of a gas through the nozzle, the gas having a temperature insufficient to cause melting of the particles in the nozzle; and injecting the particles directly into the diverging region of the nozzle at a point after the throat, entraining the particles in the flow of the gas and accelerating the particles to a velocity sufficient to result in adherence of the particles on a substrate positioned opposite the nozzle.
In another embodiment the present invention is a supersonic nozzle for a kinetic spray system comprising: a throat located between a converging region and a diverging region, the diverging region defined between the throat and an exit end; and at least one injector positioned between the throat and the exit end, the injector in direct communication with the diverging region.
In yet another embodiment the present invention is a kinetic spray system comprising: a supersonic nozzle comprising a throat located between a converging region and a diverging region, the diverging region defined between the throat and an exit end; at least one injector positioned between the throat and the exit end, the injector in direct communication with the diverging region; a low pressure powder feeder connected to the at least one injector; and a high pressure source of a heated main gas connected to the nozzle.